Figure 2-3. - Schematic symbols.

FUSE TYPES

Fuses are manufactured in many shapes and sizes. In addition to the copper fuse link already described, figure 2-1 shows other fuse types. While the variety of fuses may seem confusing, there are basically only two types of fuses: plug-type fuses and cartridge fuses.

Both types of fuses use either a single wire or a ribbon as the fuse element

(the part of the fuse that melts). The condition (good or bad) of some fuses can be determined by visual inspection. The condition of other fuses can only be determined with a meter. In the following discussion, visual inspection will be described. The use of meters to check fuses will be discussed later in this chapter.

PLUG-TYPE FUSE

The plug-type fuse is constructed so that it can be screwed into a socket mounted on a control panel or electrical distribution center. The fuse link is enclosed in an insulated housing of porcelain or glass. The construction is arranged so the fuse link is visible through a window of mica or glass.

Figure 2-4 shows a typical plug-type fuse.

Figure 2-4. - Plug-type fuses:

Figure 2-4, view A, sows a good plug-type fuse. Notice the construction and the fuse link. In figure 2-4, view B, the same type of fuse is shown after the fuse link has melted. Notice the window showing the indication of this open fuse. The indication could be either of the ones shown in figure 2-4, view B.

The plug-type fuse is used primarily in low-voltage, low-current circuits. The operating range is usually up to 150 volts and from 0.5 ampere to 30 amperes. This type of fuse is found in older circuit protection devices and is rapidly being replaced by the circuit breaker.

CARTRIDGE FUSE

The cartridge fuse operates exactly like the plug-type fuse. In the cartridge fuse, the fuse link is enclosed in a tube of insulating material with metal ferrules at each end (for contact with the fuse holder). Some common insulating materials are glass, bakelite, or a fiber tube filled with insulating powder.

Figure 2-5 shows a glass-tube fuse. In figure 2-5, view A, notice the fuse link and the metal ferrules. Figure 2-5, view B, shows a glass-tube fuse that is open. The open fuse link could appear either of the ways shown in figure 2-5, view B.

Figure 2-5. - Cartridge-tube fuse:

Cartridge fuses are available in a variety of physical sizes and are used in many different circuit applications. They can be rated at voltages up to 10,000 volts and have current ratings of from 1/500 (.002) ampere to 800 amperes. Cartridge fuses may also be used to protect against excessive heat and open at temperatures of from 165°F to 410°F (74°C to 210°C).